The notes don't define identifiers m and seq. Describe in words
instead.
While here, rename 3-structs.md to 4-structs.md.
For #65614.
Change-Id: I6690f552903d1870c70e28673c87ac84b7c5c041
Reviewed-on: https://go-review.googlesource.com/c/go/+/592195
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
